Interactions between epithelial cells and mesenchyme are required for branching morphogenesis of embryonic lung. These interactions are in part mediated by collagens and other matrix macromolecules which are deposited as a "basement membrane" at the interface between epithelial cells and the embryonic connective tissue. Recent studies suggest that type I collagen may not be required during the early phase of branching morphogenesis. However, basal lamina is necessary for branching. In addition, ultrastructural studies have demonstrated discontinuities in the basal lamina in areas of rapid epithelial proliferation. These observations suggest that the regulation of the synthesis and turnover of pericellular and basal lamina-associated collagens plays some role in modulating airway branching. Although total collagen synthesis has been measured during airway branching in vitro, little is known about the synthesis or turnover of specific collagen types during this phase of embryonic lung development. Our aim is to study the biosynthesis, extracellular deposition, and turnover of "interstitial" and "basement membrane" collagens during branching morphogenesis of embryonic mouse lung. These studies should increase our understanding of the role of collagenous proteins in modulating events associated with cellular proliferation and epithelial differentiation during airway morphogenesis.